Plasma deposition refers to any of a wide variety of processes in which a plasma is used to assist in the deposition of thin films or coatings onto the surfaces of objects. For example, plasma deposition processes are widely used in the electronics industry to fabricate integrated circuits and other electronic devices, as well as to fabricate the magnetic tapes and disks used in audio, video, and computer applications. Plasma deposition processes may also be used to apply coatings to various objects to improve or change the properties of the objects. For example, plasma deposition processes may be used to apply wear resistant coatings to machine tools, while other types of coatings may be used to increase the corrosion resistance of other items, such as bearings, turbine blades, etc, thereby enhancing their performance. In still other applications, plasma deposition may be used to apply coatings to various types of surfaces in the optics and glass industries.
In most plasma deposition processes the plasma is created by subjecting a low-pressure process gas (e.g., argon) contained within a vacuum chamber to an electric field. The electric field, which is typically created between two electrodes, ionizes the process gas and creates the plasma. In the case of a DC sputter deposition plasma process, the material (commonly referred to as the target) to be deposited on the object or substrate is connected as the negative electrode (i.e., cathode), whereas some other element, commonly the vacuum chamber itself, is connected as the positive electrode or anode. Ionized process gas atoms comprising the plasma are accelerated toward and ultimately impact the negatively charged cathode/target, dislodging or sputtering atoms from the target material. The sputtered atoms subsequently condense on various items in the chamber, including the substrate that is to be coated. The substrate is usually positioned with respect to the target so that a majority of the sputtered target atoms condense on the surface of the substrate.
Sputter deposition processes of the type described above are usually referred to as "non-reactive" processes in that the sputtered atoms deposited on the surface of the substrate do not react with other materials. However, sputter deposition processes have also been developed wherein the target material is sputtered in the presence of a reactive material (e.g., oxygen or nitrogen gas). Such "reactive" sputtering processes may be used to deposit a film comprising the sputtered target material and the reactive species. A wide variety of compounds, such as SiO.sub.2, Al.sub.2 O.sub.3, Si.sub.3 N.sub.4, and TiO, can be deposited by reactive sputter deposition processes.
While it is a relatively simple matter to connect a power supply (e.g., a DC power supply) to the electrodes and generate the plasma, it is considerably more difficult to produce and control the plasma so that the deposition process will consistently produce coatings of the desired quality. One of the main difficulties in this regard is that the plasma is generally quite unstable and is subject to significant, and perhaps rapid, variations in electrical impedance. These impedance variations may be related to a variety of factors, including the type of material being deposited, the pressure within the chamber, the strength of the magnetic field used to confine the plasma, the movement of fixtures within the chamber, and other extrinsic factors.
Regardless of their cause, the impedance variations create many problems relating to the power supply and the ability of the power supply to maintain the process at the desired operation point (e.g., power level). Consider, for example, a plasma process that is to be operated at a constant power of 8,000 watts. Suppose also that the initial impedance of the plasma is 80 ohms. At this operating point, the voltage potential between the electrodes is about 800 volts and the current in the plasma is about 10 amperes. Now, if the impedance of the plasma decreases to about 20 ohms, such as may occur when the target material is nearly depleted in a magnetron sputtering process, the voltage between the electrodes will decrease to about 400 volts while the current doubles to 20 amperes.
A power supply suitable for the foregoing process must be rated at 800 volts and 20 amperes so that it is able to provide the higher current required as the target material erodes. However, such a power supply is really twice as large as necessary in that its power rating would be 16 kilowatts, yet the process is only to be operated at 8 kilowatts. Such oversized power supplies are expensive and add to the cost of the overall process.
Partly in an effort to avoid the need to provide such oversized power supplies, some power supplies have been developed that utilize transformers with a variety of taps. The taps can be manually selected by the user to accommodate the voltage and current changes associated with the impedance changes of the plasma. Unfortunately, however, the transformer taps of such power supplies cannot be changed while the power supply is operating. Consequently, the process must be stopped so the user can change the tap to accommodate the impedance change. The process may then be restarted. Obviously, such an arrangement is cumbersome, inefficient, and adds to the overall cost of the process.
Consequently, a need remains for a power supply that can accommodate the impedance changes that commonly occur in plasma processing systems. Such a power supply should be able to deliver maximum power to the plasma over a wide range of impedances without the danger of overloading the power supply. Additional advantages could be realized if such a power supply could compensate for fairly large impedance variations, but without the need to first stop the process, then manually reconfigure the power supply.